Neptunium ( , ) is a chemical element with the symbol Np and atomic number 93. A radioactive metallic element, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, ²³⁷Np, is a by-product of nuclear reactors and plutonium production and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores due to transmutation reactions.

Characteristics

Silvery in appearance, neptunium metal is fairly chemically reactive and is found in at least three allotropes:

• α-neptunium, orthorhombic, density 20.45 g/cm³
• β-neptunium (above 280 °C), tetragonal, density (313 °C) 19.36 g/cm³
• γ-neptunium (above 577 °C), cubic, density (600 °C) 18 g/cm³

Compounds

This element has four ionic oxidation states while in solution:

• Np³⁺ (pale purple), analogous to the rare earth ion Pm³⁺
• Np⁴⁺ (yellow green)
• NpO₂⁺ (green blue)
• NpO₂²⁺ (pale pink)

Neptunium forms tri- and tetrahalides such as NpF₃, NpF₄, NpCl₄, NpBr₃, NpI₃, and oxides of the various compositions such as are found in the uranium-oxygen system, including Np₃O₈ and NpO₂.

Neptunium(V) fluoride, NpF₅, is volatile like uranium hexafluoride.

Neptunium, like other actinides, readily forms a dioxide neptunyl core (NpO₂), which readily complexes with carbonate as well as other oxygen moieties (OH⁻, NO₂⁻, NO₃⁻, and SO₄²⁻) to form charged complexes which tend to be readily mobile with low affinities to soil.

• NpO₂(OH)₂⁻
• NpO₂(CO₃)⁻
• NpO₂(CO₃)₂³⁻
• NpO₂(CO₃)₃⁵⁻

Uses

Precursor in Plutonium-238 Production

²³⁷Np is irradiated with neutrons to create ²³⁸Pu, an alpha emitter for radioisotope thermal generators for spacecraft and military applications. ²³⁷Np will capture a neutron to form ²³⁸Np and beta decay with a half life of two days to ²³⁸Pu.

\mathrm{^{237}_{\ 93}Np\ +\ ^{1}_{0}n\ \longrightarrow \ ^{238}_{\ 93}Np\ \xrightarrow[2.117 \ d]{\beta^-} \ ^{238}_{\ 94}Pu}

²³⁸Pu also exists in sizable quantities in spent nuclear fuel but would have to be separated from other isotopes of plutonium.

Weapons applications

Neptunium is fissionable, and could theoretically be used as fuel in a fast neutron reactor or a nuclear weapon. In 1992, the U.S. Department of Energy declassified the statement that Np-237 "can be used for a nuclear explosive device". It is not believed that an actual weapon has ever been constructed using neptunium. Calculations show that the critical mass is between 50 and 60 kg. As of 2009, the world production of Np-237 by commercial power reactors was over 1000 critical masses a year, but to extract the isotope from irradiated fuel elements would be a major industrial undertaking.

In September 2002, researchers at the University of California Los Alamos National Laboratory created the first known nuclear critical mass using neptunium in combination with enriched uranium, discovering that the critical mass of neptunium is around 60 kg, showing that it "is about as good a bomb material as U-235." US officials in March 2004, planned to move the nation's supply of separated neptunium to a site in Nevada.

Other

²³⁷Np is used in devices detecting high-energy (MeV) neutrons.

History

At least three times discoveries of the element 93 were falsely reported, namely as Bohemium on one hand and Ausonium one another in 1934 and then as Sequanium in 1939.

Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was first discovered by Edwin McMillan and Philip H. Abelson in the year 1940 in Berkeley, California. Initially predicted by Walter Russell's "spiral" organization of the periodic table, it was found at the Berkeley Radiation Laboratory of the University of California, Berkeley where the team produced the neptunium isotope ²³⁹Np (2.4 day half-life) by bombarding uranium with slow moving neutrons. It was the first transuranium element produced synthetically and the first actinide series transuranium element discovered.

\mathrm{^{238}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{239}_{\ 92}U\ \xrightarrow[23 \ min]{\beta^-} \ ^{239}_{\ 93}Np\ \xrightarrow[2.355 \ d]{\beta^-} \ ^{239}_{\ 94}Pu}

Occurrence

Trace amounts of neptunium are found naturally as decay products from transmutation reactions in uranium ores. Artificial ²³⁷Np is produced through the reduction of ²³⁷NpF₃ with barium or lithium vapor at around 1200 °C and is most often extracted from spent nuclear fuel rods as a by-product in plutonium production.

2 + 3 Ba → 2 Np + 3

By weight, neptunium-237 discharges are about 5 % as great as plutonium discharges and about 0.05 % of spent nuclear fuel discharges.

Synthesis

Chemically, neptunium is prepared by the reduction of NpF³ with barium or lithium vapor at about 1200 °C, however, most Np is produced in nuclear reactions:

• When an ²³⁵U atom captures a neutron, it is converted to an excited state of ²³⁶U. About 81 % of the excited ²³⁶U nuclei undergo fission, but the remainder decay to the ground state of ²³⁶U by emitting gamma radiation. Further neutron capture creates ²³⁷U which has a half-life of 7 days and thus quickly decays to ²³⁷Np.

:\mathrm{^{235}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{236}_{\ 92}U_m\ \xrightarrow[120 \ ns]{} \ ^{236}_{\ 92}U\ +\ \gamma}

:\mathrm{^{236}_{\ 92}U\ +\ ^{1}_{0}n\ \longrightarrow \ ^{237}_{\ 92}U\ \xrightarrow[6.75 \ d]{\beta^-} \ ^{237}_{\ 93}Np}

• ²³⁷U is also produced via an (n,2n) reaction with ²³⁸U. This only happens with very energetic neutrons.
• ²³⁷Np is the product of alpha decay of ²⁴¹Am.

Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure ²³⁷Np.

Role in nuclear waste

Neptunium-237 is the most mobile actinide in the deep geological repository environment.This makes it and its predecessors such as americium-241 candidates of interest for destruction by nuclear transmutation. Neptunium accumulates in commercial household ionization-chamber smoke detectors from decay of the (typically) 0.2 microgram of americium-241 initially present as a source of ionizing radiation. With a half-life of 432 years, the americium-241 in a smoke detector includes about 5 % neptunium after 22 years, and about 10 % after 43 years. After the 432-year americium-241 half-life, a smoke detector's original americium would be almost half neptunium.

Due to its long half life neptunium becomes the major contributor of the total radiation in 10000 years. As it is unclear what happens to the containment in that long time span, an extraction of the neptunium would minimize the contamination of the environment if the nuclear waste could be mobilized after several thousand years.

Isotopes

19 neptunium radioisotopes have been characterized, with the most stable being ²³⁷Np with a half-life of 2.14 million years, ²³⁶Np with a half-life of 154,000 years, and ²³⁵Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being ^236mNp (t_½ 22.5 hours).

The isotopes of neptunium range in atomic weight from 225.0339 u (²²⁵Np) to 244.068 u (²⁴⁴Np). The primary decay mode before the most stable isotope, ²³⁷Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before ²³⁷Np are element 92 (uranium) isotopes (alpha emission produces element 91, protactinium, however) and the primary products after are element 94 (plutonium) isotopes.

²³⁷Np is fissionable. ²³⁷Np eventually decays to form bismuth-209, unlike most other common heavy nuclei which decay to make isotopes of lead. This decay chain is known as the neptunium series.

References

1. "Restricted Data Declassification Decisions from 1946 until Present", accessed Sept 23, 2006

Literature

• Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
• Lester R. Morss, Norman M. Edelstein, Jean Fuger (Hrsg.): The Chemistry of the Actinide and Transactinide Elements, Springer-Verlag, Dordrecht 2006, ISBN 1-4020-3555-1.
• Ida Noddack: "Über das Element 93", in: Angewandte Chemie 1934, 47, 653–655.

External links

• WebElements.com – Neptunium (also used as a reference)
• Lab builds world's first neptunium sphere, U.S. Department of Energy Research News
• NLM Hazardous Substances Databank – Neptunium, Radioactive
• Neptunium: Human Health Fact Sheet
• C&EN: It's Elemental: The Periodic Table – Neptunium